1. Field of the Invention
The present invention relates to a technique for improving correction accuracy of shading correction in an image scanning apparatus.
2. Description of the Related Art
Conventionally, it is known that a voltage level accumulated in a CCD line sensor in an image scanning apparatus depends largely on states of a light source (when the light source is turned on and when the light source is stabilized) and ambient temperature (see, for example, JP-A-8-105357). Therefore, the image scanning apparatus performs shading correction at timing such as immediately before image scanning and adjusts a white level and a black level with respect to an image to make it possible to always acquire same image data under all conditions.
In the shading correction, a white shading correction plate is used. The correction plate is scanned by several lines and density fluctuation in an image is corrected using an average of scanned data as a reference of a white level and a black level (see, for example, JP-A-9-294207).
However, when dust or the like adheres to the shading correction plate, a value of a shading correction coefficient obtained by scanning the shading correction plate by several lines and averaging scanned data is small compared with that obtained when dust or the like does not adhere to the correction plate. This causes an image streak.
FIG. 9 is a graph showing image profile data obtained when dust adheres to the shading correction plate. In the figure, 8-bit luminance value data (on the abscissa, “0 (black)” to “255 (white)”) is shown. As shown in the figure, density is high only in a portion to which dust adheres. When such data is adopted as white reference data for shading correction, since the white reference data of a pixel in the portion to which dust adheres (and pixels around the portion) is dark, image data scanned for the portion is white and appears as a streak on an image. The same streak image also occurs when air bubbles are present in glass plates that hold the shading correction plate.
When the streak image due to dust or the like present on the shading correction plate appears as described above, it is a general practice to perform setting for changing a scanning start position on the shading correction plate. To perform data acquisition necessary and sufficient for shading correction taking into account likelihood of presence of dust, there is a limit in a reduction of a size of the shading correction plate.